feat(tui): trial an anchored composer in fullscreen mode on nightly

## Summary

Trial the existing fullscreen TUI mode on the nightly channel so the composer can remain anchored while users read earlier transcript content.

This issue follows [Discussion #3879](https://github.com/apache/maka/discussions/3879). The pinned `@earendil-works/pi-tui@0.84.2` dependency already exposes a fullscreen/alternate-screen path, so this should be evaluated as a scoped product experiment rather than a renderer rewrite.

## Experiment scope

- Run the fullscreen TUI path in the nightly channel.
- Keep the composer available while the transcript viewport scrolls independently.
- Preserve the user's reading position when new model or tool output arrives.
- Show a clear unread/new-output indication while the user is away from the bottom.
- Exercise application-owned mouse scrolling, text selection, copying, transcript search, and links.

## Evaluation questions

- Is returning to the bottom before typing a meaningful problem in real use?
- Does the anchored composer materially improve reading-while-working?
- Is application-owned selection and copying acceptable compared with native terminal behavior?
- Is losing terminal-native scrollback during and after the session acceptable?
- Should the status line remain anchored with the composer, collapse, or scroll with the transcript?
- What should happen when a user starts typing while viewing older content?

## Non-goals

- Do not make fullscreen the stable default as part of this issue.
- Do not add a permanent user-facing mode toggle before the nightly experiment produces evidence.
- Do not fork or rewrite the upstream renderer unless the existing fullscreen path proves insufficient.

## Exit criteria

Document nightly feedback and observed behavior for reading position, unread indication, selection/copy, terminal history, resizing, and small terminals. Use that evidence to decide whether to stabilize, revise, or drop the mode.

_AI assistance disclosure: Drafted and published with Codex at the author's direction._